Digital image data is data which in general describes images that are composed of pixels arranged in rows and/or columns, and whose totality results in the representation of an image. The individual pixels may in this case be described in various ways, for example broken down into components as color triples in the primary colors red, green and blue, as brightness and color difference signals Y, Cr, Cb. The image data is in this case represented by binary numerical values, so-called data words, with a specific number of digits, or word or bit length.
During the post-processing of digital image data, for example in order to correct the color values, the brightness or the contrast, the individual pixels and a correction value, which can in each case be adjusted by an operator, are supplied to a processing unit. The processing unit produces an output value at its output. Normally, both the image data and the correction value are supplied to the processing unit in binary form. In this case, the pixels and the correction values are represented by data words of a specific length, bit length or word length. Depending on the nature of the processing in the processing unit, the result of the processing, that is to say the output value, may have a greater data word bit length than each individual one of the supplied data words for the pixel value and correction value. Binary multiplication of natural numbers, in particular, results in data words with a word length which corresponds to the sum of the word lengths of the multiplicand and the multiplier. If two or more processing units are intended to be connected in series, then the word length that is required rises considerably with each processing unit. However, it is normal to connect a number of processing units in series. For example, color saturation is corrected first of all, after which the brightness is adapted. If now, for example, pixels with a word length of 32 bits and correction values with a word length of 16 bits are linked in a multiplication operation, then the resultant output value has a word length of 48 bits. If this value is processed in a further processing operation with a correction value which likewise has a word length of 16 bits, then the results of the second processing operation in fact has a word length of 64 bits. The above example shows very quickly that even a small number of processing operations carried out in series results in very great word lengths.
Nowadays, the operations carried out in professional video processing are carried out by means of logic circuits which have hardware gates in order to allow processing in real time. Real-time processing offers the user the advantage that the changes carried out by him can be seen directly on a monitor. However, it is very complex and costly to design processing units with very great word lengths. In order to keep the complexity within tolerable limits, the results are thus rounded to the word length of the input word after each processing stage. The rounding process is frequently carried out by appropriate omission of the least significant bit (LSB).
Modern video processing devices often have a number of series-connected processing units, between which the signal or the image data cannot be tapped off and which also cannot be bypassed. A situation often arises in which only one specific correction is intended to be carried out, for example the correction of a specific color value or of a specific component of a pixel, while no other corrections should be carried out. However, the image data for all of the pixels is passed through the video processing unit. In this case, the result of the processing, for example for a color value which is intended to remain unprocessed, differs from the input value as a result of the restricted computation accuracy of the processing unit, although this is also only insignificant. However, the signal is corrupted despite the only insignificant change between the output value and the input value. One object of the invention is to propose an arrangement and a method for reducing processing errors during the processing of digital image data.